Packaging for food, beverages, cosmetics, medicines, and the like require high barrier properties to oxygen and carbon dioxide to preserve the freshness of the package contents. Blends containing small amounts of high barrier polyamides, such as poly(m-xylylene adipamide), typically known commercially as MXD-6, with polyesters such as poly(ethylene terephthalate), PET, enhance the passive barrier properties of PET.
To further reduce the entry of oxygen into the contents of the package, small amounts of transition metal salts, such as cobalt salts, can be added to the blend of PET and polyamide to catalyze and actively promote the oxidation of the polyamide polymer, thereby further enhancing the oxygen barrier characteristics of the package. The active oxygen scavenging of many blends of oxygen scavenging transition metals and predominantly amine-terminated, low molecular weight polyamides with PET does not begin immediately to a significant extent. On the other hand, active oxygen scavenging of comparable compositions containing oxygen scavenging transition metal/high molecular weight commercial grade MXD-6 blends begins almost immediately, within a few days. The induction period (the period of time from the formation of the article until the time the oxygen transmission rate is significantly reduced) of many low molecular weight, amine-terminated polyamide/cobalt salt blends in PET extends well into the life cycle of a filled package so as to make these blends practically useless as active oxygen scavengers. In some cases, the induction period is so long that no significant oxygen scavenging takes place before the contents of the package are consumed, such that it no longer makes practical sense to refer to an induction period.
In addition to reducing the induction period relative to oxygen scavenging transition metal/predominantly amine-terminated low molecular weight polyamide blends in PET, it would also be yet more desirable to control the induction period. The complete elimination of an induction period is not the most preferred means for reducing the induction period. It would be more desirable to have an induction period of some length of time in order to extend the useful life of oxygen scavenging by the action of the oxygen scavenging transition metal and oxygen scavenger, referred to as the capacity to scavenge oxygen. Containers are usually stored in inventory for a period of time, open to air both inside and out of the container, before they are filled with their designated contents. During the period of storage, oxygen scavenging is not necessary. If the compositions exhibit a very short induction period, oxygen scavenging begins during the period of time when the containers are not filled, with the drawback that the useful scavenging capacity is reduced.
Accordingly, it would be desirable to control the induction period and commence the active oxygen scavenging at about the time the container is filled, typically 2-4 weeks after the article is formed, while providing a shorter induction period than in oxygen scavenging transition metal/low molecular weight amine-terminated polyamide blends in PET.